To study a part of an Electrical Flight Control System we have developed a tool-supported method dedicated to the incremental specification and validation of complex heterogeneous systems. Formal description of a system is structured in modules that interact. We combine two modular approaches that share the same view of modularity but offer complementary validation procedures: model checking and functional test generation. We have adapted these validation procedures to take care of the modular aspects of our specification. They are performed incrementally. We first consider basic modules, then the communication between modules and finally composed modules. To support our method, we have adapted existing tools, dedicated to non-modular specifications, to deal with modular constraints. These tools are integrated into a common platform to build a coherent execution environment.